Hello, Kennedy Film & TV. Short answer: I believe British, for syndication around the world.
, it states that these ran in the US in 1952-53, on NBC. I looked up the producer, Harry Alan Towers, and in that era he worked for a couple of British producers of syndicated radio shows, 'Lang-Worth' and his own 'Towers Of London', which he formed in 1946.
In the early 50s, he produced such radio shows as 'The Adventures Of Harry Lime' (known in the US as 'The Lives Of Harry Lime') and 'The Black Museum'. Both of those shows were produced in Britain, for syndication around the world, so I would assume that the same went for 'The Scarlet Pimpernel'.
If you read his short blurb on Wikipedia, he sounds like a rather unsavoury 'Harry Lime' character himself (scroll down to 'Vice Activities').